The present invention relates to a process for the preparation of a saturated carboxylic acid ester. More particularly, the present invention relates to a process for the conversion of olefins of the general formula I 
wherein, R1 is aryl, substituted aryl, naphthyl or substituted naphthyl or alkyl, R2, R3 and R4 are independently hydrogen or alkyl, to an ester of the corresponding saturated carboxylic acid of the general formula II 
wherein, R is alkyl or aryl, R1 is aryl, substituted aryl, naphthyl or substituted naphthyl or alkyl, R2, R3, R4 and R5 are independently hydrogen or alkyl, using a supported aqueous phase palladium complex catalyst.
Aryl and aliphatic saturated carboxylic acid esters have a variety of applications in industries as, anti-inflammatory drugs, fine chemicals etc. The prior art describe catalyst systems for employment in processes for the preparation of esters of carboxylic acids by the alkoxy carbonylation of corresponding olefins. The best known of such catalysts are homogeneous palladium catalysts. In general, the catalyst systems used for the alkoxycarbonylation of olefins contain a palladium source, a phosphine ligand and a hydrogen halide promoter. Alkoxycarbonylation of olefins using a catalyst system comprising PdCl2 or PdCl2(PPh3)2, excess triphenylphosphine and HCl has been found to occur only at drastic conditions such as 300-700 atm of CO pressure (Bittler et. al., Angew. Chem. Internat. Ed., 7, 1968, 329). Oi et. al. (J. Mol. Cat. A: Chem., 115, 1997, 289) have reported hydroesterification of styrene using cationic palladium complexes and which proceeds under mild conditions (20 atm, 80xc2x0 C.) to give 91-94% product yield in four hours (TOF=11 hxe2x88x921) with n: iso ratio of 60:40. Recently Seayad et al, (Ind. Eng. Chem. Res., 37, 1998, 2180, J. Mol. Catal. A: Chem., 151, 2000, 47-59) have shown enhanced reaction rates in the hydroesterification of styrene (TOF=411 hxe2x88x921) using a catalyst system comprising of Pd(OAc)2, PPh3 and p-toluene sulphonic acid with an n:iso ratio of 35:64. A major disadvantage of these homogeneous catalytic processes was the difficulty in separation of the catalyst from the product and its recycle. An important method for heterogenizing such homogeneous catalysts is the application of two-phase systems comprising an aqueous phase containing water-soluble organometallic catalysts and a water immiscible phase (U.S. 31812; Kuntz E. G. CHEMTECH 17, 1987, 570; EP 0107006; B. Cornils, W. A. Herrmann (Eds.), Aqueous-Phase Organometallic Catalysis, Wiley-VCH, 1998, Weinheim). In this case, separation of the organometallic catalyst from the organic reactants and products is greatly simplified due to the insolubility of the catalyst in water immiscible phase.
Supported Aqueous Phase Catalysis (SAPC) (U.S. Pat. Nos. 5,736,980, 5,935,892) is another method to heterogenize homogeneous catalysts. Here, the catalytic material consists of a thin film of water containing a metal complex catalyst spread over a high-surface-area inorganic support, such as silica. The molecular catalyst is immobilized via the solvent i.e. water, reactants and products being in the organic phase. The main advantages of the SAP catalysts are easy catalyst recovery and good activity because of the high interfacial area, a property particularly sensitive with sparingly water-soluble reactants (J. P. Arhancet, M. E. Davis, J. S. Merola, Be. Hanson, Nature, 339, 1989, 454; K. T. Wan, M. E. Davis, Nature, 370, 1994, 449; K. T. Wan, M. E. Davis, J. Catal., 148, 1994, 1).
The object of the present invention therefore is to provide an improved process for the preparation of carboxylic acid esters by the alkoxycarbonylation of corresponding olefins using supported aqueous phase palladium complex catalysts.
It is another object of the invention to provide a process for the preparation of a saturated carboxylic acid ester that overcomes the drawbacks of the prior art enumerated above.
It is another object of the invention to provide a process for the preparation of a carboxylic acid ester that results in good yield and selectivity to carboxylic acid esters.
It is a further object of the invention to provide a process for the preparation of a carboxylic acid ester that provides simple and efficient catalyst separation and recycle.
It is observed in the invention that the use of supported aqueous phase palladium complex catalysts provide an improved catalyst for the carbonylation of olefins to corresponding saturated carboxylic acid esters. The use of such a catalyst gives good yields of carboxylic acid esters under mild reaction conditions with easy separation and reuse of the catalyst.
Accordingly, the present invention provides a process for the preparation of a carboxylic acid ester of the general formula II, 
wherein R is alkyl or aryl, R1 is aryl, substituted aryl, naphthyl or substituted naphthyl or alkyl, R2, R3, R4 and R5 are independently hydrogen or alkyl, said process comprising reacting an olefin of the general formula I 
wherein, R1 is aryl, substituted aryl, naphthyl or substituted naphthyl or alkyl, R2, R3, and R4 are independently hydrogen or alkyl, in the presence of an alcohol and an organic solvent and a supported aqueous phase palladium complex catalyst, in presence or absence of a protonic acid and an alkali metal halide, under carbon monoxide atmosphere, at a temperature ranging between 30 to 130xc2x0 C., for a period ranging between 1-72 hours, at pressures ranging between 50 to 1500 psig, cooling the reaction mixture to ambient temperature, depressurising the reactor, flushing the reaction vessel with inert gas, separating the catalyst by filtration and removing the solvent and isolating the compound of formula II.
In one embodiment of the present invention, the catalyst used comprises a watersoluble palladium complex with or without free sufonated phosphine ligand.
In another embodiment of the invention, the water soluble palladium complex used are selected from the group consisting of any of Pd(II) and Pd(0) compounds of the type PdII(OAc)2P2, PdIIP2Cl2, PdIIP3Cl]+Clxe2x88x92, Pd0P2, Pd0P3 and mixtures thereof; wherein P is a sulfonated phosphine ligand; or a complex of the general formula III 
wherein R1, R2 and R3 are substituents on the phosphine ligand and selected from the group consisting of alkyl, aryl, arylalkyl, cycloaliphatic, at least one of which carries a sulfonic acid, and salts thereof, X is aryl or alkyl sulfonato or aryl or alkyl carboxylate or formato or halides such as Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, 
is an anionic chelating ligand containing a N donor and Oxe2x88x92 group.
In a further embodiment of the invention, said anionic chelating ligand is selected from the group consisting of 8-hydroxyquinoline, 2-hydroxypyridine, (2-hydroxyethyl)pyridine, pyridil-2-, pyrezyl-2, piperidyl-2-, piperzyl-2-, quinolyl-2-, isoquinolyl-1- and isoquinolyl-2-carboxylates, particularly pyridyl-2-carboxylate, piperidyl-2-carboxylate and 8-hydroxyquinoline.
In another embodiment of the present invention, the support used for the preparation of the supported aqueous phase palladium complex catalyst is selected from porous or nonporous silica.
In yet another embodiment the sulfonated phosphorous ligand used for the preparation of the supported aqueous phase catalyst comprises a sulfonated mono phosphines.
In a further embodiment of the invention, the sulfonated mono phosphine is selected from the group consisting of tris(sodium 3-sulfonatophenyl)phosphine (TPPTS), phenylbis(sodium-3-sulfonatophenyl)phosphine (TPPDS), diphenyl (sodium-3-sulfonatophenyl)phosphine (TPPMS), methylbis(3-sulfonatophenyl)phosphine, cyclohexylbis(sodium-3-sulfonatophenyl)phosphine, isopropyl bis(sodium-3-sulfonatophenyl)phosphine, dimethyl(sodium-3-sulfonatophenyl)phosphine and dicyclohexyl(3-sulfonatophenyl)phosphine.
In another embodiment the amount sulfonated phosphine ligand used per gram mole of palladium for the preparation of the supported aqueous phase palladium catalyst may be 1-20 moles, preferably 2-6 moles.
In another embodiment of the present invention, the protonic acid used for the alkoxycarbonylation reaction is selected from a hydro halic acids or a protonic acids.
In a further embodiment of the invention, the hydrohalic acid is selected from the group consisting of hydrochloric acid, hydrobromic acid and hydro iodic acid.
In a further embodiment of the invention, the protonic acid is selected from the group consisting of para toluene sulphonic acid, methane sulphonic acid, triflouromethane sulphonic acid, formic acid, oxalic acid, acetic acid and trifluoro acetic acid.
In yet another embodiment the halide source used for the alkoxycarbonylation reaction a halide salt or a hydrohalic acid.
In a further embodiment of the invention, the halide salt is selected from the group consisting of lithium chloride, sodium chloride, potassium chloride, lithium iodide, lithium bromide, sodium bromide, sodium iodide, potassium bromide, potassium iodide, tetrabutylammonium chloride, tetrabutylammonium bromide and tetrabutylammonium iodide.
In another embodiment of the present invention, the hydrohalic acid is selected from the group consisting of hydrochloric acid, hydrobromic acid and hydroiodic acid.
In another embodiment the alcohol used for the alkoxycarbonylation reaction is selected from the group consisting of methanol, ethanol, n- or iso propanol, n-, iso- or tert-butanol, higher alcohols and phenols.
In yet another embodiment the organic solvent for the alkoxycarbonylation reaction is selected from the group consisting of cyclohexane, benzene, toluene, xylenes, petroleum ether, hexane, heptane and decane.
In another embodiment the concentration of catalyst is one mole of catalyst for every 50 to 50000 moles of substrate.
In a further embodiment of the invention, the concentration of the catalyst is 1 mole of catalyst for every 100 to 10000 moles of substrate, more preferably one mole of catalyst for every 150 to 5000 moles of substrate.
In still another embodiment the amount of halide source if used per gram mole of catalyst is in the range of 1 to 50 moles, preferably 5 to 10 moles.
In another embodiment the amount of acid source if used per gram mole of catalyst is in the range of 1 to 50 moles, preferably 5 to 10 moles.
In a feature of the invention, the reaction is carried out in a stirred reactor with the improved catalyst employed with a suitable solvent in presence of carbon monoxide.